Donald SterlingNBA Lawyer Authenticates Audio

4/29/2014 1:00 AM PDT

Two NBA lawyers left the law firm representing V. Stiviano Monday ... and sources familiar with the situation tell TMZ Sports ... the lawyers are now convinced -- the racist audio of Donald Sterling is REAL.

NBA lawyer David Anders and another NBA lawyer went to the law office of Mac Nehoray Monday afternoon. Stiviano was also there for the purpose of authenticating the audio.

We're told there was NO doubt by anyone the recording is the real deal. The NBA lawyers heard the 15 minutes that have been made public, but not the remaining 45. And we're told ... based on conversations along with analysis ... they're satisfied about authenticity.

Neither Stiviano nor Anders had comments for our photog as they left the building.

TMZ Sports has also learned ... people who have heard the entire recording say there is "NO DOUBT" Donald Sterling KNEW he was being recorded.